The mechanisms and rates of mercury methylation in the Florida Everglades are of great concern because of potential adverse impacts on human and wildlife health through mercury accumulation in aquatic food webs. We developed a new PCR primer set targeting hgcA, a gene encoding a corrinoid protein essential for Hg methylation across broad phylogenetic boundaries, and used this primer set to study the distribution of hgcA sequences in soils collected from three sites along a gradient in sulfate and nutrient concentrations in the northern Everglades. The sequences obtained were distributed in diverse phyla, including Proteobacteria, Chloroflexi, Firmicutes, and Methanomicrobia; however, hgcA clone libraries from all sites were dominated by sequences clustering within the order Syntrophobacterales of the Deltaproteobacteria (49 to 65% of total sequences). dsrB mRNA sequences, representing active sulfate-reducing prokaryotes at the time of sampling, obtained from these sites were also dominated by Syntrophobacterales (75 to 89%). Laboratory incubations with soils taken from the site low in sulfate concentrations also suggested that Hg methylation activities were primarily mediated by members of the order Syntrophobacterales, with some contribution by methanogens, Chloroflexi, iron-reducing Geobacter, and non-sulfate-reducing Firmicutes inhabiting the sites. This suggests that prokaryotes distributed within clades defined by syntrophs are the predominant group controlling methylation of Hg in low-sulfate areas of the Everglades. Any strategy for managing mercury methylation in the Everglades should consider that net mercury methylation is not limited to the action of sulfate reduction.

Gradients in phosphorus (P) removal and storage were investigated over 6 years using mesocosms (each consisting of three tanks in series) containing submerged aquatic vegetation (SAV) grown on muck and limerock (LR) substrates. Mean inflow total P concentrations (TP) of 32 g L(-1) were reduced to 15 and 17 g L(-1) in the muck and LR mesocosms, respectively. Mesocosm P loading rates (mean=1.75 gm(-2) year(-1)) varied widely during the study and were not correlated with outflow TP, which instead varied seasonally with lowest monthly mean values in December and January. The mesocosms initially were stocked with Najas guadalupensis, Ceratophyllum demersum, and Chara zeylanica, but became dominated by C. zeylanica. At the end of the study, highest vegetative biomass (1.1 and 1.4 kg m(-2) for muck and LR substrates) and tissue P content (1775 and 1160 mg kg(-1)) occurred in the first tank in series, and lowest biomass (1.0 and 0.2 kg m(-2)) and tissue P (147 and 120 mg kg(-1)) in the third tank. Sediment accretion rates (2.5, 1.9 and 0.9 cm yr(-1) on muck substrates), accrued sediment TP (378, 309 and 272 mg kg(-1)), and porewater soluble reactive P (SRP) concentrations (40, 6 and 4 g L(-1)) in the first, second and third tanks, respectively, exhibited a similar decreasing spatial trend. Plant tissue calcium (Ca) near mesocosm inflow (19-30% dry weight) and outflow (23-26%) were not significantly different, and sediment Ca was also similar (range of 24 to 28%) among sequential tanks. Well-defined vegetation and sediment enrichment gradients developed in SAV wetlands operated under low TP conditions. While the mesocosm data did not reflect deterioration in treatment performance over 6 years, accumulation of P-enriched sediments near the inflow could eventually compromise hydraulic storage and P removal effectiveness of these shallow systems.